The present invention relates to a relatively simple instrument for determining the degree of tension of a tensioned sheet of limp material. While the apparatus is primarily intended for determining the amount of tension in a screen employed in "silk screening" operation, it could be employed in any application where it was desired to determine, or measure, tension of a tensioned sheet of limp material to a given standard, e.g., the tension of a drum head.
While a silk fabric was originally employed in the so-called "silk screening" process for applying designs of colored pigment to a surface of fabric, paper, etc., sheets of various other materials are now employed alternatively for such screens, for example, phosphor bronze or stainless steel metal fabric or fabrics made of synthetic materials such as nylon and/or polyester. In the silk screen operations the fabric, of whatever kind, is placed in a frame and tensioned. See, for example, U.S. Pat. Nos. 2,565,218 and 3,391,635. For many operations it is highly desirable to have a specific degree of tension in the fabric, not necessarily in absolute figures but in comparative figures as related to a predetermined standard. One commercially available device for this purpose comprises a support which spans the frame holding the fabric and rests thereon. A one pound ball is mounted so as to rest on the fabric adjacent its center and a dial gauge mounted on the support and connected to the ball exhibits the extent to which the ball depresses the fabric as a result of the weight thereof.
One problem with such prior art devices is that they do not distinguish between the tension in the warp direction and the tension in the weft direction. If the tension in one of these directions is very high, the tension in the other direction may be relatively low without that fact being indicated by such a prior art device. An important feature of the present invention is that it provides a degree of directional sensitivity whereby information can be obtained as to the tension in the warp direction and the tension in the weft direction.
A significant feature of the present invention is that it is a relatively small, compact unit. It occupies little space when not in use. It may be employed to detect the tension in a sheet of virtually every size that might be employed in a silk screening operation. The components thereof are easily and simply manufactured by conventional production techniques or are commercially available, off-the-shelf, type items.
The metal fabric, as discussed above, deflect less when under a given tension and a given deflecting force is applied than do the natural or synthetic fabrics. Thus the present invention incorporates a range or sensitivity switch to accommodate the apparatus to such characteristics.
Other advantages and objects of the present invention will be apparent from the following description.